Generally, IPM motors, which use expensive permanent magnets, are expensive, but are more efficient than induction motors. For this reason, IPM motors are widely used, for example, for driving motors and power generating motors for hybrid vehicles and electric vehicles, and also motors for home electric appliances, various machine tools and industrial machines.
An iron core of an IPM motor is composed of a stator and a rotor. Since an AC magnetic field is directly applied to the iron core on the stator side through windings, the iron core on the stator side must have high magnetic permeability and also high volume resistivity so as to reduce iron loss. Therefore, electromagnetic steel sheets with soft magnetic characteristics improved by the addition of Si to ultra-low-carbon steel are used for the iron core on the stator side (see, for example, Patent Documents 1 and 2).
On the other hand, since a permanent magnet is embedded in the iron core on the rotor side, this iron core mainly acts as a yoke to increase magnetic flux density. The iron core on the rotor side is slightly affected by the AC magnetic field generated from the stator side, but this influence is limited. Therefore from the standpoint of characteristics, it is not necessary to use electromagnetic steel sheets, which are advantageous for the iron loss characteristic, for the iron core on the rotor side. However, the same electromagnetic steel sheets as used for the stator side are also usually used for the iron core on the rotor side because the product yield of the electromagnetic steel sheets decreases and the production costs of the motor increase when the electromagnetic steel sheets are used only for the stator.
When an IPM motor is to be installed in a vehicle, the IPM motor needs to be reduced in size because of the need to reduce the size and weight of the vehicle. In this case, the rotational speed of the rotor is increased in order to obtain a motor output (torque) equal to or greater than that of a conventional motor despite the reduction in size. The efficiency of a motor generally improves as the rotational speed of the rotor increases. However, in an IPM motor, an induced electromotive force is generated on the stator windings by the rotation of the embedded permanent magnets. This induced electromotive force increases with the increase in the rotational speed. Where the electromotive force exceeds the input voltage, the motor can no longer rotate. Therefore, in an IPM motor, field-weakening control, which generates a magnetic flux from the stator side in a direction to cancel the magnetic flux of the permanent magnets and suppresses the induced electromotive force, is performed when the motor is operated in a high-speed rotational range, as indicated, for example, in Patent Document 3. The field-weakening control enables the operation in a high-speed rotational range, but decreases the motor torque because power is used for cancelling the magnetic flux of the permanent magnets. Patent Document 3 indicates that the amount of electricity to be used for the field-weakening control is decreased by improving the shape of the magnets.
Meanwhile, even if the IPM motor is reduced in size, there is a problem that where rotational speed of the rotor is increased so as to obtain a torque equal to or higher than that in a conventional motor, the centrifugal forces that act upon the permanent magnets embedded in the rotor increase thereby damaging the rotor. To prevent this damage, it is preferred that a material with a high yield strength be used for the rotor. For example, a non-orientated electromagnetic steel sheet (35A300) containing about 3% Si has a yield strength after magnetic annealing of approximately 400 N/mm2. Therefore in the case of a comparatively large IPM motor with a rotor diameter of 80 mm or more, the limit of the rotational speed at which damage is not caused is about 20,000 rpm, although the specific value somewhat differs depending on the structure of the rotor. A variety of research has been conducted to increase the yield strength of iron cores based on electromagnetic steel sheets, but still the yield strength is at most about 780 N/mm2. As a method for suppressing damage to a rotor core caused by high-speed rotation, for example, Patent Document 4 suggests using steel sheet with high strength and high saturation magnetic flux density, rather than electromagnetic steel sheet, as a material for the rotor core.
Patent Document 1: Japanese Patent Application laid-open No. 2005-133175
Patent Document 2: Japanese Patent Application laid-open No. 2005-60811
Patent Document 3: Japanese Patent Application laid-open No. 2000-278900
Patent Document 4: Japanese Patent Application laid-open No. 2009-46738